The present invention relates to digital wireless communications systems and more particularly, to a method and apparatus for a signal power control in such systems.
Over the past few years, various wireless architectures have been developed in response to user demands for systems which can offer high-rate data communications. Recently for example, broadband wireless access (BWA) systems have become of interest to the wireless industry for their ability to supply high speed multimedia data services.
In a wireless communication system such as a BWA network, it is common to speak of a point-to-multipoint architecture in which a base transceiver station (BTS) is positioned at the center of a service area (normally called a cell) and communicates over the air with multiple customer premises equipment (CPE) units located within the same cell. The BTS is usually fixed and designed with a single receiver for servicing all of the CPE units present in the cell. The CPE units can be designed to have fixed locations within the cell or alternatively be portable and free to roam.
Typically, the direction of communication from the BTS to the CPE units is called the downstream direction and the reverse direction of communication is referred to as the upstream direction. In order to send separate downstream data streams to multiple CPE units at the same time, the data is usually multiplexed. There is presently various multiplexing methods used for downstream communications. For example, downstream data is often transmitted by frequency division multiple access (FDMA) by assigning a distinct downstream carrier frequency to each CPE unit.
More commonly however, downstream data destined for several CPE units is multiplexed in time and is transmitted at a common forward carrier frequency. This can also apply to upstream communications with upstream data originating from several CPE units multiplexed in time and transmitted at another common carrier frequency. A cellular system that has these characteristics is usually referred to as a time-division multiple access (TDMA) system.
In order to multiplex several data streams into a single TDMA stream, the time structure of a communications link is divided into scheduling periods having a fixed number of time slots per scheduling period. The CPE units can only access the upstream link if an opportunity is granted in a scheduling period by the BTS. When an opportunity is given to a particular CPE unit to transmit on the upstream link, the CPE unit initially notifies the BTS of the amount of upstream bandwidth it requires for its transmission. This bandwidth on-demand process causes upstream transmissions to occur in bursts.
The upstream and downstream data is typically transmitted in dedicated frequency channels also referred to as radio links. Because radio link transmissions are carried over air, channel conditions associated with radio links are much worse than those related to physical wires. Transmission impairments such as variable propagation path loss, impulse noise, fading, interference and most notably rain attenuation often occur as a result of environmental variations to corrupt and attenuate transmitted signals. These impairments dramatically reduce transmission reliability and in some situations may even cause link failures. As a result, the maintenance of reliable radio links is extremely important for a wireless system.
A key aspect in maintaining a reliable radio link between CPE units and a BTS in a wireless system is the ability to counteract signal power variations caused by transmission impairments. At the BTS, an automatic gain control (AGC) circuit will generally be used in the BTS receiver to adjust the signal power received and compensate for variations caused by transmission impairments so that signals can be received at the BTS at a relatively constant power level. In a point-to-multipoint TDMA system, the BTS receiver AGC must be designed to operate rapidly and on a burst-by-burst basis to efficiently service all of the CPE units present in the cell.
However, conventional AGCs are not well suited for high-frequency signals transmitted in bursts such as upstream radio signals transmitted from CPE units to a BTS. In current systems, these burst signals typically operate at high symbol rates in excess of 5 Megabauds with bursts of varying sizes and separated by idle periods of varying durations. Conventional AGCs are not suited for such high-frequency and bursty transmissions from different CPE units and would not be able to react quickly and adequately to counteract power variations in each signal burst received. Therefore, it would be desirable to provide a burst-to-burst signal power compensation scheme at the BTS to efficiently service all of the CPE units present in a cell.
It would also be desirable to have a large BTS receiver dynamic range to counteract wide power variations in the signals received from the CPE units. These variations are common in wireless because radio signals are often seriously impaired or corrupted during transmission as a result of changes in the environment such as, for example, rainfalls. A large BTS receiver dynamic range is also desirable to address the well-known near-far design issue which arises as a result of large variations in propagation path loss caused by user mobility and changing distances between the CPE units and the BTS.
However, conventional AGCs are not currently designed with a sufficiently large dynamic range and cannot counteract wide power variations in the signals received from the CPE units. Designing an AGC with a dynamic range large enough to accommodate transmissions from the CPE units would be prohibitively expensive and unnecessarily complex.
To alleviate some of the shortcomings of conventional AGCs, it is known to use a transmit signal power control at each CPE unit. With this technique, an optimal (desired) receive signal power level is set near the middle of the BTS receiver dynamic range. When the power of a received signal varies away from the optimal level, the BTS instructs each CPE unit to adjust its transmit power such that the signal power received at the BTS can be maintained at the optimal level.
Presently, transmit power controls are used in various communications systems to help reduce the impact of transmission impairments and improve link reliability. Unfortunately, for a radio signal transmitted from a CPE unit to a BTS which is impaired or corrupted by changing environmental conditions, the signal power received at the BTS may fall outside the BTS receiver dynamic range well before the CPE transmit power can be adjusted to compensate. This is particularly true of radio signals transmitted at high frequencies which are more likely to suffer from environmental variations than low frequency signals.
Thus, there is a need to provide a signal power control for wireless systems to counteract transmission impairments affecting radio signals transmitted and maintain reliable radio link connectivity.
The present invention addresses these issues and to this end provides a methodology and apparatus to mitigate the present limitations in this art.
The invention provides a signal power control method and apparatus for efficiently controlling in a wireless system the power level of a received signal transmitted over a radio link so as to counteract transmission impairments caused by environment variations and maintain the reliability of the radio link. In order to efficiently and rapidly control the signal power of a received radio signal, the signal power control of this invention serves to control the transmit power used for transmission and adjust the position of the receiver dynamic range so that the received signal can always be optimally received.
In a preferred embodiment, the invention is incorporated in a broadband wireless access (BWA) time division multiplex access (TDMA) system to maintain the reliability of an upstream radio link connecting a base transceiver station (BTS) with multiple customer premises equipment (CPE) units. In the preferred embodiment, the signal power of upstream signals received at the BTS is controlled with two power control loops. A first control loop is used for adjusting the transmit power at each CPE unit and a second control loop is used to adjust the position of the BTS receiver dynamic range so that the upstream burst signals can all be received.
According to the invention, each control loop relies on a communication protocol such as the international telecommunications union (ITU) R112 local multipoint distribution service (LMSD) standard as a means to exchange power control messages between the BTS and the CPE units.
Advantageously, with the CPE transmit control loop and the BTS receiver range control loop, the invention can effectively counteract impairments brought about by environmental changes which may afflict upstream signals during transmission. By compensating for transmission impairments, the upstream connectivity between the BTS and each CPE unit can be reliably maintained.
With the combined control of the CPE transmit power and the BTS receiver dynamic range, the invention can also be advantageously used for providing power control in wireless systems where receivers exhibit a narrow receive power dynamic range.
Yet another advantage of using a CPE transmit control loop and a BTS receiver range control loop is that the received power of signals operating at high frequency and in bursts such as upstream signals in a BWA TDMA system can be efficiently controlled.